1. Field of the Invention
The present invention relates to a group III nitride semiconductor having a reduced dislocation density and a manufacturing method thereof, and further relates to a group III nitride independent substrate separated from a base substrate.
2. Description of the Related Art
A group III nitride semiconductor has been put to practical use as a material for manufacturing a light emitting device, an electronic device and the like, and has been paid attention for the use in a region which could not be covered by a conventional semiconductor material.
Normally, in order to manufacture those devices, a group III nitride semiconductor layer is epitaxially grown on a substrate crystal. When Si, GaAs or the like is applied, a wafer having a large diameter and a low defect density is commercially manufactured as the substrate crystal, which enables to manufacture a lattice-matching type device. However, when the group III nitride semiconductor is applied, since no homoepitaxial substrates having good quality and being inexpensive do not exist, a heterogeneous substrate such as a sapphire substrate having a different lattice constant, a thermal expansion coefficient, and the like has to be normally used in substitution for the homoepitaxial substrate under the present situation. For this reason, a dislocation density of about 109 to 1010/cm2 is normally introduced into a group III nitride semiconductor crystal grown on the sapphire substrate.
A blue LED (Light Emitting Diode) can realize a high efficient light emission specifically even under the situation of the high dislocation density, but, it is found that this is contributed by a composition fluctuation of In in a light emitting layer. However, a blue-violet laser with an emission wavelength of 405 nm used as a light source of a next-generation DVD is operated in an incommensurably higher current injection density than the LED, so that a dislocation existing in a light emitting stripe and being a non-light emission center increases, which creates a problem regarding a life deterioration in which a light emission efficiency rapidly decreases. Further, regarding a light emitting element in an ultraviolet region, since there is a limit in an adding amount of In on the ground of mixed crystal composition, as the wavelength of the element becomes shorter, more problems in the efficiency and the decrease of the operating life due to the dislocation to be the non-light emission center are created. Furthermore, also in a bipolar type electronic device element, an increase of a leak current, a deterioration of element characteristic, and the like caused by the existence of the dislocation become problems. Therefore, a reduction in the dislocation density is a big task (“Widegap Semiconductor Opto-electronic Devices” editorially supervised by Kiyoshi TAKAHASHI, edited and written by Fumio HASEGAWA and Akihiko YOSHIKAWA, published by Morikita Publishing Co., Ltd. (March, 2006)).
Meanwhile, there is a need to improve characteristics of the various devices, and to realize a high-power, for instance, there is a need to improve a heat release performance. This becomes an important subject in future investigation especially in an LED used for illumination and for a head lamp of a car and in a high-frequency/high-power device. Specifically, it is required to reduce a heating value by improving an efficiency in an operation section, and to efficiently diffuse the generated heat. To satisfy the former requirement, a measure such that a reduction in a crystal defect and an optimization of an element structure can be done, and to satisfy the latter requirement, a measure such that the optimization of the element structure similarly as above, a reduction in the thickness of a base substrate by a grinding, a separation of a crystal layer from a low-heat conductivity substrate to transfer it to a high-heat conductivity substrate, or the use of the high-heat conductivity substrate can be done.
Heat conductivities in the vicinity of a room temperature of typical semiconductor substrate materials are 150 W/mK (Si), 50 W/mK (GaAs), 42 W/mK (sapphire), and 450 W/mK (SiC), and since the sapphire substrate normally used as the group III nitride semiconductor has a low-heat conductivity, there is proposed, as the aforementioned measure, a method to separate a grown crystal layer from the sapphire substrate using a laser lift-off method. Further, if GaN (230 W/mK) and AlN (330 W/mK) having good heat conductivities can be used as the substrates, it is expected to obtain an effect of reducing the crystal defect and to be advantageous in terms of heat release, but, there is a problem that there exists no substrates being inexpensive with good quality under the present situation (W. S. Wong et al., “Damage-free separation of GaN thin films from sapphire substrates” Appl. Phys. Lett. 72 (1998) P. 599, and “IMEC improves GaN HEMTs” Compound Semiconductor, October (2005) P. 16).
In order to reduce the dislocation density of the group III nitride semiconductor crystal grown on the sapphire substrate, an improvement of a group III nitride buffer layer, a control of a propagation of a threading dislocation from a base substrate by a lateral growth on an insulating film called an ELO (Epitaxial Lateral Overgrowth), a control of a propagation of the threading dislocation from the base substrate by a method called PENDEO-epitaxy method in which a group III nitride type layer is disposed on an upper surface of a convex portion of a concave and convex processing substrate and is grown in a hollow portion from a side surface of the substrate in a lateral direction, and so on are proposed. Although depending on a growth film thickness, the dislocation density can be reduced by about one to two digits with the use of those methods. Further, in GaN, since the dislocation is eliminated because of a reaction of each dislocation along with the progress of formation of the crystal layer and thus the dislocation density is lowered, there has been developed a thick film crystal having a low dislocation density using an HVPE (Hidride Vapor Phase Epitaxy) method capable of performing a high-speed epitaxy. If the thick film crystal is grown to have a thickness of about several hundreds of μm to 1 mm, the dislocation density can be decreased to a value of a digit of 107 to 106/cm2, so that it has been developed and manufactured especially for the use of the independent substrate and a template substrate. However, to obtain the independent substrate, the aforementioned laser lift-off method is applied, specifically, GaN on an interface is decomposed by a nanosecond pulse irradiation of an excimer laser of 248 nm from a back surface side of the sapphire substrate to thereby separate the GaN from the substrate. In this case, there are a lot of problems in the yield such that the entire surface cannot be perfectly peeled off and a crack is generated, which becomes a main cause for increasing a cost (Amano, et al., “Effect of low-temperature-deposited layer on the growth of group III nitrides on sapphire” Applied Physics vol. 68 (1999) P. 768, A. Sakai, et al., “Defect structure in selectively grown GaN films with low threading dislocation density” Appl. Phys. Lett. 71 (1997) P. 2259, K. Linthicum et al., “Pendeoepitaxy of gallium nitride thin films” Appl. Phys. Lett. 75 (1999) P. 196, and S. K. Mathis et al., “Modeling of threading dislocation in growing GaN layer” J. Crystal Growth 231 (2001) P. 371).